1. Field of the Invention
The present invention relates to an optical filter applicable to an optical fiber transmission network for realizing long-distance large-capacity transmission of signal light components having wavelengths different from each other (to be referred to as wavelength-multiplexed signal light hereinafter) and an optical transmission system having the optical filter.
2. Related Background Art
Conventionally, large-capacity high-speed communication such as image communications or long-distance communication such as international communication has been performed using an optical fiber transmission network. From the viewpoint of large-capacity high-speed communications, these recent communication techniques mainly employ wavelength division multiplexing optical transmission (WDM optical transmission) for transmitting wavelength-multiplexed signal light through an optical transmission line. In addition, from the viewpoint of long-distance communication, optical amplifiers for amplifying signal light components are sometimes arranged in the optical fiber transmission line.
An Er-doped optical fiber amplifier (EDFA) generally used as the optical amplifier can amplify wavelength-multiplexed signal light components having wavelengths within the 1.55 .mu.m wavelength band at once.
The gain spectrum, being wavelength dependant is not flat but changes depending on the wavelength of the wavelength-multiplexed signal light. Generally, the gain spectrum has a tendency to have the gain at a maximum near the center wavelength of the wavelength band of the wavelength-multiplexed signal light and becomes smaller as the wavelength shifts from the center wavelength. The wavelength band of the wavelength-multiplexed signal light is defined as a difference between a maximum wavelength and a minimum wavelength of the wavelength-multiplexed signal light.
Signal light components having wavelengths different from each other, which are outputted from the EDFA, have different intensities depending on the wavelengths. When a number of EDFAs are arranged in the optical fiber transmission line, the signal light components have a predetermined intensity at the transmission terminal of the optical fiber. At the reception terminal of the optical transmission line, however, the intensity ratio between the signal light components becomes higher. In some cases, one of a plurality of signal light components may have a sufficient intensity at the reception terminal while the remaining signal light components may have lower intensities and therefore cannot be received.
Studies have been made to arrange, in the optical fiber transmission line, an optical filter (equalizer) having a transmission spectrum (loss spectrum) for canceling the gain spectrum of the EDFA such that the gain spectrum of the entire transmission line is flattened. The transmission spectrum of this optical filter has reverse characteristics to those of the gain spectrum. The transmission loss is maximum near the center wavelength within the band of wavelength-multiplexed signal light and becomes smaller as the wavelength shifts from the center wavelength. For such an optical filter, application of a fiber grating having an appropriately designed period has been examined.
For example, a technique is disclosed in A. M. Vengsarkar et al., "Long-period fiber gratings as gain-flattening and laser stabilizing devices" in which two fiber gratings having grating periods different from each other are cascaded and used as the optical filter, and the gain spectrum of the EDFA is canceled by the transmission spectrum of this optical filter to reduce the difference between the maximum gain and the minimum gain of the gain spectrum of the entire transmission line in the band of wavelength-multiplexed signal light to 0.2 dB or less.